The flocculation of suspended matter in water to enhance clarification rates is an important aspect of industrial and municipal water treatment. Flocculation is the agglomeration of coagulated colloidal and finely divided suspended matter by physical mixing or chemical coagulant aids. Polymeric organic coagulants such as cationic polyamines and high molecular weight polyacrylamides have been used to aid in flocculation and are often used in combination with inorganic coagulants such as lime, alum, ferric chloride, ferrous sulfate, ferric sulfate and sodium aluminate.
Cationic copolymers are used in such applications as flocculating agents, paper treating agents, and the like. Conventionally known cationic copolymers include copolymers of metal or ammonium salts of dialkylaminoalkyl(meth)acrylates, and Hofmann degradation or Mannich reaction products of polyacrylamides. Recently, considerable research effort has been devoted to developing vinylamine containing copolymers by copolymerizing and modifying copolymers of N-vinylamide.
While many polymeric materials can be used effectively as flocculants for clarifying water streams containing suspended matter, only certain of these copolymers are effective as drainage/retention aids in the manufacture of paper or paperboard. Retention is a term used in papermaking to denote the extent to which the pulp fibers and other additives which are added to the furnish are retained in the finished paper. The retention of pulp fibers, fines, sizing agents, fillers and other additives in the paper sheet during its formation in a paper making machine is an important problem. A retention aid generally acts by increasing the flocculating tendency of the pulp fibers and additives to inhibit their loss during drainage through the paper machine wires or screens.
Numerous factors affect the efficiency of retention aids including 1) variables in the furnish such as pH, consistency, temperature, type of pulp fiber (e.g., fiber length, degree of refining, etc.), and white water recirculation (e.g. degree of system closure), 2) conditions of the wire or screens such as wire mesh size, machine speed, etc. and 3) factors relating to the additives such as the dosage amount of additives, order of additives, form, shape and density of particles and ionic balance.
Drainage is another papermaking requirement, that often conflicts with retention, and requires a rapid reduction in water content of an aqueous pulp suspension in the sheet forming areas of a paper machine. Aqueous pulp suspensions contain more than 99% water. To convert an aqueous pulp suspension to a finished paper sheet requires a rapid reduction in water content to a level of about 6%. Drainage rates are dependant upon numerous factors including the arrangement of the drainage elements in the paper making machine, (e.g., arrangement of free drainage areas vis-a-vis vacuum assistance area), characteristics of the wires, screens or fabric, furnish characteristics (e.g. freeness, additives, etc.), furnish thickness, temperature, furnish consistency and wire speed. Suitable retention/drainage aids must not only inhibit the undue loss of fibers and additives, but they must also promote rapid drainage of water from the pulp suspension. Numerous retention/drainage aids are known and are available to paper makers.
EP 235,893 describes the use of a combination of organic, substantially linear synthetic polyacrylamide copolymers and bentonite to improve drainage/retention. Suitable organic copolymers are those which provide a bridging mechanism for flocculation. Suitable copolymers are disclosed as having charge densities from 0.35 to 2.5 mEq/gm, and molecular weights above 500,000, preferably above 1 million and often above 5 million, most preferably in the range 10 to 30 million or more. However, these high molecular weight copolymers are provided to paper makers in the form of a solid material which is slow to dissolve in aqueous systems. In addition, high molecular weight copolymers are very shear-sensitive, which present significant handling and quality control problems. For example, since the copolymers must be pre-dissolved in an aqueous solution, extensive mixing is required to assure complete dissolution. However, since these copolymers are sensitive to shearing, mixing procedures which are adequate to assure dissolution often destroy the desirable high molecular weight characteristics as well as overall uniformity of the final polymeric dispersion.
U.S. Pat. No. 4,749,444 discloses a process for production of paper and cardboard by adding to the paper stock a three component mixture comprising an activated bentonite, a cationic polyelectrolyte having a charge density not less than 4 mEq/gm and a high molecular weight acrylamide or methacrylamide copolymer having an average molecular weight from 1 to 20 million. This reference discloses that if less than all three components are used, i.e., if only bentonite and cationic polyelectrolyte are used, the drainage of the paper stock is poor or if only bentonite and high molecular weight copolymer are used, the paper stock flocculates to such an extent that satisfactory sheet formation is not ensured.
Linhart and Auhorn, in Das Papier, No. 10A, 1992, pp. v38-v45, discloses the use of polyvinylamines as a new class of copolymers for paper production.
U.S. Pat. No. 5,098,521 discloses a process for paper and board production which uses a paper stock containing anionically charged foreign substances by adding to the paper stock N-vinylamide/vinylamine copolymers having less than 10% vinylamine units and having K values of not less than 130 (as determined according to H. Fikentscher).
U.S. Pat. No. 4,774,285 discloses vinylamine containing copolymers by copolymerizing N-vinylformamide with an ethylenically unsaturated monomers such as vinyl acetate, vinyl propionate, C.sub.1 to C.sub.4 alkyl vinyl ethers, the esters, nitriles and amides of (meth)acrylic acid and N-vinylpyrrolidone and then hydrolyzing the copolymers. These copolymers are used in papermaking to increase the strength of paper.
U.S. Pat. No. 4,808,683 discloses copolymers containing vinylamine, N-vinylformamide and N-monosubstituted or N,N-disubstituted acrylamide for use as flocculating agents, drainage aids and paper strength increasing agent.
U.S. Pat. No. 4,957,977 and U.S. Pat. No. 5,064,909 disclose vinylamine containing copolymers by copolymerizing N-vinylformamide and (meth)acrylonitrile and then hydrolyzing the resulting copolymers with an acid. These copolymers are useful as flocculating agents and paper strength increasing agents.
U.S. Pat. No. 5,037,927 discloses copolymers of N-vinylformamide and alkyl(meth)acrylate and their hydrolyzed products.
The above patents disclose copolymers of N-vinylformamide with a nonionic comonomer. Upon hydrolysis, the N-vinylformamide component in the copolymers is converted into a cationic vinylamine component. Thus, the cationic charge on the resultant copolymers will be derived solely from the vinylamine component and accordingly, the charge density of the copolymers is greatly affected by the pH of the medium into which they are added. That is, at low pH, the vinylamine copolymers become protonated and have a labile high cationic charge density and conversely, at high pH, vinylamine copolymers are not fully protonated, and the copolymers will have a substantially reduced cationic charge density. None of the above patents disclose copolymers of N-vinylformamide which derive their cationic charge from pH independent cationic comonomers.